The present invention relates to new and improved "B" stage (intermediary) systems of fiber reinforced phenol-formaldehyde sheet molding compositions (SMC), which are sufficiently flexible to be easily and usefully formed and/or moldable into predetermined and desired shapes or use forms.
This invention also relates to the curing of these "B" staged compositions, after forming or fitting into molds where finished products are produced, which exhibit improved physical characteristics such as superior fire retardation, strength, hardness, rigidity, and some flexibility useful for fitting or implacement in areas of construction or use. Also relating to this invention is the formulation of the SMC to be essentially non-shrinking during the curing process.
An advantage to the use of a "B" stage system is in its properties which allows manufacturers to ship and/or store the molding compositions in cohesive sheets, rolls, folds and other forms prior to molding in a separate molding operation into desired cured, rigid articles.
The prior art of simple phenolic molding compounds has a long history. Detailed description of this history can be found in "The Chemistry of Phenolic Resins" by Martin, John Wiley and Sons, Inc., 1956, and its cited references. Also, the "Encyclopedia of Polymer Science and Technology" Vol. 10, John Wiley and Sons, Inc., New York, describes the early history and general uses of phenolic resin based general purpose, high impact strength, heat resistant materials and molding compounds used for special electrical insulation applications. The manufacture of phenolic molding compounds encompasses a hot compounding operation in which phenol-formaldehyde resin is fluxed and intimately mixed with such dry ingredients as hexamethylene tetramine, wood flour, lime, stearic acid and nigrosine. These dry ingredients are preblended and fed to heated rolls where a compounded sheet is formed as the resin melts and in so doing wets and disperses the fillers through the sheet to form a homogeneous mixture of the filler and resin. The warm sheet of compounded resin is cooled, crushed, and ground to the desired particle sizes to form a molding powder for use in compression molding. The molding powder may be poured directly into the mold cavities or may be cold pressed to produce shaped pre-forms which may then be subsequently heat cured to form the finished article.
One U.S. Patent to Kevin M. Foley, et al, No. 3,944,515 discloses a type of fiber reinforced phenolic SMC based upon the incorporation of Portland cement-type material which, upon forming the composition into a predetermined shape such as flat sheet and then thermosetting, produces flat sheets of completely cured hard sheets of considerable flexural strength, flexural modulus, and tensile strength. The phenolic condensate produced has a shelf life sufficiently long, under ordinary conditions, that sheet molding compound produced from it within four hours is satisfactory. The reactive Portland cement present in the composition assists the curing of the resin into an insoluble condition. In the practice of this invention, the mix composition is maintained under heat and pressure in a predetermined molded shape and this shape is maintained while curing it into the finished infusible article.
The art of SMC based on polyester resins (apart from the phenolic kinds) is extensively developed and is particularly exemplified by the disclosures set forth in the U.S. Pat. Nos. 3,975,353; 3,994,853; 3,925,300; and 3,988,290.
The sheet molding compounds (SMC) of the present invention are fibrous reinforced mats impregnated with compositions containing "B" stage (partially polymerized) phenolic resin systems compounded with catalysts, fillers, and other like compounding materials. This composite is formed in a partially curable flexible sheet which can be fabricated or molded by heat and/or pressure into hard, infusible shaped articles.
In the phenolic SMC compositions of this invention, it has been discovered that reactive cementaceous material can be eliminated to provide flexible rather than inflexible "B" stage compositions that can be stored or transported in rolls, flats, or folded shapes prior to the use of the "B" stage compounds in a final molding operation which, by heat and/or pressure the curing of the compounds is effected, thus producing the desired rigid articles.
One of the advantages of the system of this invention is to permit resin formulators to store or ship resin products in cohesive and stable sheet or bulk form to secondary manufacturers for convenient local molding and final curing into infusible final products.
An object of the present invention is to provide a flexible reinforced sheet molding composition (SMC) containing a partially polymerized phenolic resin system that can be stored and shipped in sheets, rolls, or flat folded shapes which, upon molding under heat and/or pressure, can be finally cured to form rigid finished articles.
A further object is to provide a glass fiber reinforced "B" stage sheet molding compositions that upon molding under heat and/or pressure, produce finished articles having excellent surface characteristics, high flexural strength and low flame emission characteristics.
A still further object is to provide a method for forming from phenolic resin systems a "B" stage fiber reinforced sheet molding compound.
Another object is to provide phenolic resin based compositions that contain fibrous reinforcement which may be applied over fiber reinforcing mats in place in a mold.
A still further object is to provide a method of continuously forming flexible, partially cured fiber reinforced sheet molding compounds that can be molded under heat and/or pressure to rigid finished articles.